Prototype 1 - Detector using fluorescent tubes (Very Unstable)

Note: This prototype is too unstable and not recommended.

First Prototype demonstrated at Dorkbot Meeting

My first detector prototype was not that dissimilar to the CERN example, except the fluorescent tubes are placed between three metal plates. The outer plates are connected together by bolts and connected to the Negative rail of the supply and the centre plate is insulated by the fluorescent tubes and connected to the Positive rail of the supply. So far I have found the best result with small 6W fluorescent tubes is around 650V DC

First Prototype Built

Like the CERN example, when a muon flys through the fluorescent tube, the gas inside ionizes due to the high voltage field across the plates. As a result of the ionization the resistance across the plates will fall slightly and so it should be possible to measure this as a change in current flow in the high voltage source.

Schematic of first tests.

The reason for two rows of fluorescent tubes is to sense the crude presents of coincidence occurring in the top and bottom rows of fluorescent tubes due to a muon flying through both. I'm speculating that the resistance in the detector should be half compared with only one row detecting something, due to terrestrial noise. If the output is feed into a data logger and also speculate that over time the difference between cosmic and terrestrial detections could be filtered.

Off on a tangent again.

I couldn't help noticing the similarity with flash tubes and other types of gas filled trigger electronics like a Thyratron thermionic valve. Basically these tubes are biased at a voltage below ionization and when a high voltage trigger is applied briefly in the gas path between the Cathode and Anode, the gas to within the tube ionizes, the resistance to falls rapidly between the Cathode and Anode and like SCR current flows until power is removed.

Consequently I tried biasing the individual fluorescent tubes using their standard electrodes with a DC voltage somewhere below their point of ionization ~70V through a high impedance RC network. The RC network preventing sustained ionization, so producing just a pulse.

Schematic of trigger experiments various component values where tried

However, to my surprise I got quite the opposite, as I measured a voltage spike across the electrodes rather than a dip and so it would seem biasing may not be required as a strong positive spike can be clearly observed on a CRO without any biasing.

Schematic of experiment first tried and demonstrated at a Dorkbot meeting.

Summary

Nevertheless, even though "something" is causing clear observable pulses on a CRO in all variations tested above, it is difficult to confirm they are actually due to Cosmic Rays or Terrestrial Radiation over something like coronal discharges within the tube itself.

All attempts to find RFI sources have drawn a blank as pulses disappeared when the high voltage supply was switched off, other Electrical Interference has also been ruled out shielding inside a metal box. I also ruled out the supply itself without the detector and could not find any other interference sources.

I should also note that early in my building and testing of these ideas, I found that most HV supplies I built had quite allot of noise or ripple present, specially the type often recommended for Geiger Counters, so I spent quite a bit of time trying to eliminate this, with improved voltage regulation and a good bank of capacitors.

Results

Tests with an xray source have confirmed the system dose detect radiation, however once the gas inside the tube ionizes, spurious pulses re-occur randomly after, which I suspect is caused by photons being emitted inside the tube causing new avalanches occur. Increasing the impedance of the high voltage supply and placing a discharge resistor in circuit does reduce some of the problem, but this also decreases the output signal. Also I have moved away from using the filament electrodes of the lamp, although this also detects radiation successfully with a high output voltage it also significantly increases the problem of oscillation and other spurious pulses.

So I have moved to a new improvement prototype with better coupling and RFI controls see: Prototype 2 for details.

Prototype 2 - Detector using fluorescent tubes (improved still unstable)

Note: This project is still very much a work in progress and there are several issues to iron-out, if you have any questions or comments to contribute please feel free to contact me.

I found an easier and more effective way to capacitively couple the High Voltage supply to the gas inside the fluorescent tube using self-adhesive copper tape either side of the tube.

Prototype 2 initial test rig connections

This allows easier wiring and mounting multiple tubes within an enclosure and may also allow for the building of a larger array detector. First I have built a simple two tube detector and after some experimentation I have found a few more refinements to implement into the next prototype coming soon.

Simple two tube prototype in testing

I am currently building a unit Detector Using Geiger–Müller Tubes as a test unit to use as a standard to measure the performance of the Fluorescent Tube Detector against. This will greatly help resolve issues and fine tune the design...

Electrical characteristics of the fluorescent tube detector.

The resistance of the gas within a fluorescent lamp is virtually an open circuit at room temperature until a sufficient electric field is applied where the gas atoms absorb enough energy to emit a free electrons resulting in ionization.

Once the gas is ionised the electrical characteristics of the fluorescent lamp changes and begins to conduct electricity with a negative differential resistance and so more current flows. Consequently, the electrical resistance of the fluorescent lamp drops dramatically from many mega ohms to hundreds of ohms.

Ionisation of the gas within fluorescent lamp occurs at around 300 to 600V depending on the size of tube, environmental factors and the type of gas mixture used by the manufacturer this is usually a low pressure mercury vapour and a mixture of either argon, xenon, neon, or krypton.

In the cosmic ray (muon) detector the electrodes are not in direct contact with the gas inside the tube but capacitive coupled through the glass wall as identified in Diagram 1.

Where C1 represents the capacitive effect of these electrodes and R1 in series with VD represent the gas inside the tube. CP represents the parallel capacitance of the overall coupling effect from the two electrodes across the entire tube.

When a high voltage is applied E (~600 to 1200V) across the circuit it creates an electrostatic field within the gas and when this charge reaches sufficient energy the gas ionises (emitting a flash of light) causing the field to collapses and discharging C1 until the charge can build once again. If not quenched rapidly or the voltage applied is too high the circuit forms a basic relaxation oscillator and the process repeats itself rapidly proportional to the time constant of C1 and R1.

In the detector configuration in below the voltage E is set at a point just below ionisation.

!he 1M Resistor forms a series LC network with the tube capacitance C1 ensuring that only a small amount of energy is stored which can be easily quenched shortly after being triggered. The 4 10Meg Resistors also play an important role by reducing parallel capacitance, preventing the voltage across the tube C1 to run away due to the high impedance of the supply causing oscillation and also ensures a zero bias to the filaments of the fluorescent tube reducing the production of spurious pulses and discharges within the tube.

Consequently, when a Muon passes through the tube, some of the gas molecules are ionised, creating positively charged ions, and electrons. The strong electric field created by the electrodes accelerate the ions towards the negative side of the tube wall and the electrons towards the positive side of the tube walls. The ion pairs should gain sufficient energy to ionise the gas further through collisions along the way, creating an avalanche of charged particles and discharging the energy in C1 resulting in a short pulse of current which can be measured across the 1 Meg resistor as a negative travelling pulse of voltage, not dissimilar to a Geiger-Müller tube.

Issues using Fluorescent Tubes

I have been testing a number of different design variations and have identified the following issues.

1) Power supply requires good filtering and regulation - Completed High Voltage Regulated Power Supply
2) Tubes vary in voltage requirements from one tube to another even between the same make, model and age
3) Oscillation is a problem as the supply voltage and/or coupling plate surface area increase
4) Internal filament electrodes must be insulated, even loose coupling increases oscillation and spurious pulses
5) Coupling plates should be positioned back 1cm from the tip of the internal filament electrodes
6) Oscillation occurs as the circuit forms a basic relaxation oscillator

Although oscillation is an unwanted artifact, it would also seem there is a point before oscillation begins where the tube increases sensitivity to radiation as the voltage increase and approaches a point where oscillation begins. However, radiation (cosmic or terrestrial) is also what first triggers the tube to jump into an unstable state before free oscillation begins.

Nevertheless, I will investigate this further to see how oscillation could be regulated through some form of negative feedback or quenching circuit as this may yield useful results.

Compton Scattering
Tests using a Geiger–Müller array detector have revealed a problem which will equally effect fluorescent tubes detector called Compton Scattering, this is where an interaction between charged electrons within the detector and high energy photons result in the electron being given part of the energy, causing a recoil effect of a high energy photon into the adjacent detector causing a false coincidence detection. In other words this causes cross-talk interference between Detector Tubes

Fluorescent Tube Detector Electronics

The electronics component of the detector is an important part of a Cosmic Ray Telescope and comes in two main parts.

Detection Circuit

One of the problems with building a detector using fluorescent tube is that although it functions very much like a Geiger-Müller tube the voltage is higher; the signal has a very high output impedance and is capacitive coupled. So the following considerations needed to be included in the circuit design:

1. The amplifier section must tolerate high voltages and have a high input impedance.
2. Capacitive coupling introduces an AC signal characteristics and amplitude variations, so the output needs shaping through a Schmitt trigger for better stability and noise immunity.
3. The high voltage power supply must have good filtering and be regulated to decrease amplitude variations and introduction of noise.

Coincidence Circuit

The main problem with the physical muon detector is that in essence it is a radiation detector and along with muons showering down from the skies there also equal amounts of terrestrial radiation present in the environment including the detector itself. Although this is in very small quantities it is sufficient to make it difficult to discriminate between a terrestrial or cosmic events.

However, a muon does have sufficient energy to pass through the physical detector easily, whereas terrestrial radiation will not. Therefore anything detected in two or more detectors placed one above the other, simultaneously (coincidence) is almost certainly a cosmic event. Consequently, having electronics that can measure coincidence across two or more detectors is essential.

To do this task the output of the Schmitt Trigger is feed into a logic gate eg: AND Gate where the output could be captured by a data logger recording counts over time or other devices.

Pulse Shortening
Tests using other more reliable detector systems such as Geiger–Müller Tubes have shown that a simple coincidence detector using a AND Gate by itself is problematic due to the response and decay time (Pulse Width) of the detector when an ionising particle has been detected.

Consequently, the wider the Pulse Width the greater the number of false positives. The means a pulse shorting or quenching circuit is also needed to shorten the Pulse Width to a period closer to the expected flight time of the Muon between tubes, but not too narrow that the electronics cannot measure relative coincidence. Some improvement might also be achieved by spacing the tubes further apart, but this also has the negative effect of decreasing the aperture of the detector.